1. Field of the Invention
The present invention relates to the field of diagnosis and treatment of glaucoma. More specifically, the invention provides methods and compositions for diagnosing and treating glaucoma and for identifying agents potentially useful for the treatment of glaucoma.
2. Description of the Related Art
There are a number of ocular conditions that are caused by, or aggravated by, damage to the optic nerve head, degeneration of ocular tissues, and/or elevated intraocular pressure. For example, “glaucomas” are a group of debilitating eye diseases that are a leading cause of irreversible blindness in the United States and other developed nations. Primary Open Angle Glaucoma (“POAG”) is the most common form of glaucoma. The disease is characterized by the degeneration of the trabecular meshwork, leading to obstruction of the normal ability of aqueous humor to leave the eye without closure of the space (e.g., the “angle”) between the iris and cornea (Vaughan, D. et al., (1992)). A characteristic of such obstruction in this disease is an increased intraocular pressure (“IOP”), resulting in progressive visual loss and blindness if not treated appropriately and in a timely fashion. The disease is estimated to affect between 0.4% and 3.3% of all adults over 40 years old (Leske, M. C. et al. (1986); Bengtsson, B. (1989); Strong, N. P. (1992)). Moreover, the prevalence of the disease rises with age to over 6% of those 75 years or older (Strong, N. P., (1992)).
Glaucoma affects three separate tissues in the eye. The elevated IOP associated with POAG is due to morphological and biochemical changes in the trabecular meshwork (TM), a tissue located at the angle between the cornea and iris. Most of the nutritive aqueous humor exits the anterior segment of the eye through the TM. The progressive loss of TM cells and the build-up of extracellular debris in the TM of glaucomatous eyes leads to increased resistance to aqueous outflow, thereby raising IOP. Elevated IOP, as well as other factors such as ischemia, cause degenerative changes in the optic nerve head (ONH) leading to progressive “cupping” of the ONH and loss of retinal ganglion cells and axons. The detailed molecular mechanisms responsible for glaucomatous damage to the TM, ONH, and the retinal ganglion cells are unknown.
Twenty years ago, the interplay of ocular hypertension, ischemia and mechanical distortion of the optic nerve head were heavily debated as the major factors causing progression of visual field loss in glaucoma. Since then, other factors including excitotoxicity, nitric oxide, absence of vital neurotrophic factors, abnormal glial/neuronal interplay and genetics have been implicated in the degenerative disease process. The consideration of molecular genetics deserves some discussion insofar as it may ultimately define the mechanism of cell death, and provide for discrimination of the various forms of glaucoma. Within the past 10 years, over 15 different glaucoma genes have been mapped and 7 glaucoma genes identified. This includes six mapped genes (GLC1A-GLC1F) and two identified genes (MYOC and OPYN) for primary open angle glaucoma, two mapped genes (GLC3A-GLC3B) and one identified gene for congenital glaucoma (CYP1B1), two mapped genes for pigmentary dispersion/pigmentary glaucoma, and a number of genes for developmental or syndromic forms of glaucoma (FOXC1, PITX2, LMX1B, PAX6).
Thus, each form of glaucoma may have a unique pathology and accordingly a different therapeutic approach to the management of the disease may be required. For example, a drug that effects the expression of enzymes that degrade the extracellular matrix of the optic nerve head would not likely prevent RGC death caused by excitotoxicity. In glaucoma, RGC death occurs by a process called apoptosis (programmed cell death). It has been speculated that different types of insults that can cause death may do so by converging on a few common pathways. Targeting downstream at a common pathway is a strategy that may broaden the utility of a drug and increase the probability that it may have utility in the management of different forms of the disease. However, drugs that effect multiple metabolic pathways are more likely to produce undesirable side-effects. With the advent of gene-based diagnostic kits to identify specific forms of glaucoma, selective neuroprotective agents can be tested with the aim of reducing the degree of variation about the measured response.
Glaucoma is currently diagnosed based on specific signs of the disease (characteristic optic nerve head changes and visual field loss). However, over half of the population with glaucoma are unaware they have this blinding disease and by the time they are diagnosed, they already have irreversibly lost approximately 30-50% of their retinal ganglion cells. Thus, improved methods for early diagnosis of glaucoma are needed.
Current glaucoma therapy is directed to lowering IOP, a major risk factor for the development and progression of glaucoma. However, none of the current IOP lowering therapies actually intervenes in the glaucomatous disease process responsible for elevated IOP and progressive damage to the anterior segment continues. This is one possible reason why most patients become “resistant” to conventional glaucoma therapies. Thus, what is needed is a therapeutic method for altering (by inhibiting or even reversing) the disease process.